DECUSSATION OF THE OPTIC NERVE. 513 



vision supplied what was required, so long as it was exercised upon the 

 object ; but as soon as this guiding influence was withdrawn, the strong- 

 est will could not sustain the muscular contraction. Again, in the 

 production of vocal sounds, the nice adjustment of the muscles of the 

 larynx, which is requisite to produce determinate tones, can only"be~ 

 effected in obedience to a mental conception of the tone to be uttered ; 

 and this conception cannot be formed, unless the sense of hearing has 

 previously brought similar tones to the mind. Hence it is that persons 

 who are born deaf are also dumb. They may have no malformation of 

 the organs of speech ; but they are incapable of uttering distinct vocal 

 sounds or musical tones, because they have not the guiding conception, 

 or recalled sensation, of the nature of these. By long training, and by 

 efforts directed by the muscular sense of the larynx itself, some persons 

 thus circumstanced have acquired the power of speech ; but the want of 

 sufficiently definite control over the vocal muscles is always very evident 

 in their use of the organ. 



906. Quitting now the functions of the Sensory Ganglia, we have 

 briefly to notice certain peculiarities in the characters of the Nerves to 

 which they serve as the centres. And of these peculiarities, there is 

 one of a very remarkable nature, which is common to the three nerves 

 of special sense, namely,' the Olfactive, Optic, and Auditory; that 

 they are not in the least degree endowed with common sensibility ; so 

 that they may be cut, stretched, pinched, &c., without producing the 

 least pain. Consequently, the ordinary sensibility of the surfaces they 

 supply is entirely due to the branches of the Fifth pair, which are dis- 

 tributed upon them ; and we may have a loss of either the general or 

 special sensibility of any of the organs of sense, without the other being 

 affected, save indirectly. Again, we do not find that irritation of these 

 nerves produces any other purely reflex movements, than such as are 

 connected with the operations of the organs of sense, in which they 

 respectively originate. Thus the Olfactory nerve cannot, by any irrita- 

 tion, be made to excite a reflex movement ; the only reflex action that 

 can be excited by irritating the Optic nerve, is contraction of the 

 Pupil ; and the regulation of the tension of the Membrana Tympani (if, 

 as is probable, this is effected by the motor power of the Facial nerve, 

 excited by impressions made upon the organ of sense), appears to be 

 the only reflex action to which the Auditory nerve can minister. 



907. There is a further peculiarity, of a very marked kind, attend- 

 ing the course of the Optic nerves ; this is the crossing or decussation 

 which they undergo, more or less completely, whilst proceeding from 

 their ganglia to the eyes. In some of the lower animals, in which the 

 two eyes (from their lateral position) have entirely different spheres of 

 vision, the decussation is complete ; the whole of the fibres from the 

 right Optic ganglion passing into the left eye, and vice versd. This is 

 the case, for example, with most of the Osseous Fishes (as the cod, 

 halibut, &c.) ; and also, in great part at least, with Birds. In the 

 Human subject, however, and in animals which, like him, have the two 

 eyes looking in the same direction, the decussation seems less complete ; 

 but there is a very remarkable arrangement of the fibres, which seems 

 destined to bring the two eyes into peculiarly consentaneous action. 



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